Hydraulic systems for machines such as excavators, earthmovers, and the like, typically include a pump control having a high pressure cut-off valve. The high pressure cut-off valve functions to reduce the displacement of the pump so that pump flow is at substantially zero whenever the pump output pressure approaches the opening pressure of a main relief valve of the system, to prevent large power losses occurring when high pressure fluid is vented to a tank through the relief valve. Many operators like this feature because it reduces power losses, and, thus saves fuel.
However, some operators believe that the machine is not really working hard unless the engine lugs down due to fluid flow across the relief valve during heavy loading. Also, after the displacement of the pump has been reduced, some time lag will exist before high pump flow is reinstated. Thus, some machine hydraulic systems are not provided with high pressure cut-off valves, and some operators of machines with high pressure cut-off systems prefer to switch the systems off when operating under heavy loads, to avoid the above described problems.
With either a high pressure cut-off capability enabled or bypassed, it is still desirable to provide a load sensing capability, that is, the capability to operate the system under high pressure conditions without automatic substantial reductions in pump displacement. It is also desirable to have the capability to operate the system under high loading conditions without substantial reductions in pump displacement and the resultant time lags before pump pressure rises again.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.